The purpose of this research is a study of the factors which control the balance between RNA transcription and replication during the life cycle of a negative strand RNA virus. On the basis of preliminary evidence obtained in the rhabdovirus system a model is put forward which proposes that the phosphoprotein of the vesicular stomatitis virus (VSV) nucleocapsid functions in regulating the balance between viral RNA replication and transcription. The proposal outlines a method to test this hypothesis which involves a VS viral mutant that possesses an altered phosphoprotein which is associated with an abnormal pattern of RNA synthesis. In addition a role for the phosphoprotein in the synthesis and processing of functional mRNA is postulated.